The following background information is provided to assist the reader to understand one of the many environments in which the invention will typically be used. Upon reading this document, the reader will appreciate that the invention may also be applied or adapted to environments other than that described below.
FIG. 1 illustrates one cylinder of an electronically controlled multi-cylinder engine that is equipped with a mechanism capable of varying the timing of the opening and closing of the intake and exhaust valves. While the engine 1 is operating, air at atmospheric pressure is drawn into an inlet 2 through a filter 3 and into an intake duct 4. The incoming air then flows into a throttle body 5 in which is disposed a throttle valve 6. The throttle valve 6 typically takes the form of a rotatable plate.
Controlled by an electronic throttle control (ETC) system, the throttle plate 6 has its position adjusted regularly to allow an amount of air appropriate to present conditions to pass through the throttle body 5 and thereafter into an intake manifold 7. The throttle control system typically features a pedal sensor 8, a throttle position sensor (TPS) 9, a motor 10 and an electronic control module (ECM) 11. The pedal sensor 8 enables the ECM 11 to monitor the position of the accelerator pedal, and thus to determine whether the driver wants the vehicle to maintain, increase or decrease torque. The TPS sensor 9 enables the ECM 11 to monitor the angular position that the throttle plate 6 occupies in the throttle body 5. Pursuant to prior art algorithms, the ECM 11 uses the input from these sensors, as well as other sensors, such as those shown in FIG. 2, to control the engine 1 so that it delivers the desired torque according to the conditions under which the vehicle is operating. In doing so, the ECM 11 controls via motor 10 the position of the throttle plate 6, and thus the quantity of air that is drawn into the intake manifold 7.
From the intake manifold 7 the incoming air then passes to an intake duct 12 that leads to the cylinder 13. Meanwhile, fuel from a fuel tank 14 is pumped via a pump 15 through a pipe 16 to a fuel injector 17. According to known practice, the ECM 11 uses data from several sensors to calculate the injector pulse width, i.e., the electrical signal that the ECM 11 uses to activate the fuel injector 17 for a time appropriate to current conditions. Activated via a drive circuit, the fuel injector 17 injects the precise amount of fuel into the intake duct 12. There, the fuel mixes with the inlet air coming from the intake manifold 7.
As noted in greater detail infra, a cam timing mechanism drives the intake valve 18 to the open position in timed relationship with the intake cycle of cylinder 13. During the intake cycle, a low pressure condition develops within the combustion chamber 19 due to the downward movement of a piston 20 within the cylinder 13. The low pressure draws the fuel-air mixture from the intake duct 12 past the intake valve 18 and into the combustion chamber 19. For the subsequent compression cycle, the action of the cam timing mechanism(s), as noted infra, closes the intake and exhaust valves 18 and 21 at the top of the cylinder 13. During the compression cycle itself, as is well known, the upward movement of the piston 20 compresses the air-fuel mixture in the combustion chamber 19 of the cylinder 13.
During the combustion cycle, the fuel-air mixture is ignited and exploded to produce power. Operating according to the spark sequence controlled by an electronic spark timing system, the ECM 11 causes the air-fuel mixture to be ignited in the combustion chamber 19. More specifically, the combustion cycle is initiated, at the appropriate time, by a spark driven across the spaced electrodes of a spark plug 22. The explosive force of the combustion drives the piston 20 downward within cylinder 13. The downward thrust of the piston 20 is imparted via connecting rods 23 as a torque upon a crankshaft 24 of the engine 1. Combined with the torque it receives from the other pistons in the engine 1, the crankshaft 24 drives the wheels and the accessory loads, etc., of the motor vehicle, as is generally understood in the art.
For the exhaust cycle, a cam timing mechanism drives the exhaust valve 21, at the appropriate time, to the open position. During the exhaust cycle, the upward movement of the piston 20 forces the exhaust gases produced by combustion past the exhaust valve 21 and into an exhaust manifold 25. An exhaust pipe 26 then channels the exhaust gases to a catalytic converter 27. A catalyst within the converter 27 aids the oxidization of unburned constituents, such as carbon monoxide (CO) and hydrocarbons (HC), and the reduction of nitrogen oxides (NO.sub.X). From the converter 27, the purified exhaust gases are conveyed typically through a muffler and then through a tail pipe to atmosphere.
The ECM 11 monitors and controls the operation of the engine 1 through many data sensors, switches and control devices, some of: which are shown in FIGS. 1 and 2. In addition to the pedal and TPS sensors 8 and 9, the data sensors include an intake air temperature (IAT) sensor 28, a coolant temperature sensor (CTS) 29, a manifold absolute pressure (MAP) sensor 30, a vehicle speed sensor (VSS) 31, an oxygen (O.sub.2 2) sensor 32, and an engine speed (RPM) sensor 33. On some vehicles, additional data sensors are used. These include a wide range air-fuel (WRAF) sensor 34, a barometric pressure (BARO) sensor 35, and a mass air flow (MAF) sensor 36. The devices and subsystems that the ECM 11 controls, include the electronic throttle control system, the electronic spark timing system, the fuel injection system and the cam timing mechanisms.
The data sensors generate electrical signals, typically in analog form, indicative of the parameters they are intended to measure. The IAT sensor 28 typically measures the temperature of the air in the inlet 2 of the engine 1. The CTS sensor 29 senses the temperature of the coolant that flows in channels 37 around the cylinders to keep the engine cool. The MAP sensor 30 measures the absolute air pressure in the intake manifold 7. The VSS, sensor 31 generates a pulse representing the actual speed of the vehicle. The O.sub.2 sensor 32 is typically mounted to the exhaust system downstream of the converter 27 so that its head lies exposed to the stream of exhaust gases produced by the engine 1. It senses the free oxygen concentration in the exhaust gases, and conveys a corresponding signal to the ECM 11. Typically exposed to the exhaust gases upstream of the converter 27, the WRAF sensor 34 measures the air-fuel ratio. It is used on some vehicles to measure directly the ratio of air to fuel for purposes of controlling the delivery of fuel to the engine 1. The ECM 11 uses the signals from the O.sub.2 and WRAF sensors 32 and 34 to control more precisely the fuel-air mixture to achieve stoichiometry. This correction process is known as closed loop operation.
On vehicles equipped with BARO and MAF sensors, the BARO sensor 35 measures the pressure of the ambient air and provides data to the ECM 11 as to pressure changes due to altitude and weather. The MAF sensor 36 measures the rate at which the air mass flows into the intake manifold 7. For vehicles not equipped with a BARO sensor 35, the ECM 11 is programmed to estimate the barometric pressure using data from various other sensors according to well-known practice. For vehicles not equipped with a MAF sensor 36, the ECM 11 estimates the air mass flow rate using data from the various other sensors, as is also known in the art.
The analog signals generated by the data sensors are conveyed to the ECM 11 where an A/D converter 40 converts them into digital signals. This conversion is necessary because the central processing unit (CPU) 41 of the ECM 11 can only manipulate digital information. Along with the input received by the interface (I/F) 42, the digital sensor data is conveyed to input registers in the ECM 11. Using the data it reads from the registers, the CPU 41 not only performs the mathematical computations and logic functions necessary to calculate inter alia the spark timing, the cam timing and the proper fuel-air mixture, but also provides control signals through drive circuits 43-47, The CPU 41 performs all of its functions according to the programming code stored in its associated memory devices. The memory devices include random access memory (RAM) 48 and read only memory (ROM) 49 inclusive of programmable ROM (PROM). The CPU 41 uses RAM 48 to temporarily store information such as the data received from the data sensors, the diagnostic codes and the results of its calculations. The ROM 49 is where the calibration data and fuel delivery algorithms are typically stored along with various lookup tables and control algorithms that collectively constitute the programming code. The elements in the ECM 11 are connected to one another through a system bus 50 containing address, data and control buses.
Used primarily to maintain the engine 1 at idle, the idle speed control (ISC) system includes the ECM 11 and an idle air control (IAC) valve 51. The IAC valve 51 is situated in a flow path parallel to that through the throttle body 5. Upon closure of the throttle plate 6 and feedback from the VSS sensor 31 indicating the vehicle has stopped, the ISC system compares the actual engine speed with a target engine speed it derives according to known practice. Based on the difference between the target and actual values, the ISC system controls the IAC valve 51 via drive circuit 43 so as to adjust the rate at which air flows into the engine 1 and thereby attain the target idle speed.
The electronic spark timing (EST) system includes the ECM 11, the RPM sensor 33 and a distributor module 55. The RPM sensor 33 generates a pulse for every 30 degrees that the crankshaft 24 rotates, thereby providing a measure of the speed, or revolutions per minute (rpm), at which the engine 1 is operating. Through the data sensors, the ECM 11 monitors the speed and other operating conditions of the engine 1, and, from those parameters, calculates the proper spark timing. According to the spark timing sequence, the ECM 11 then directs the distributor module 55 via drive circuit 44 to distribute to each of the spark plugs 22, at the appropriate time, the energy required to achieve combustion.
The fuel injection system includes the ECM 11 and the fuel injector 17. Operating according to known principles, the ECM 11 uses data from several sensors to calculate the target air-fuel ratio. The mass of intake air per engine revolution is calculated from the mass flow rate of intake air measured by the MAF sensor 36 and the engine speed detected by the RPM sensor 33. Alternatively, it may also be estimated using data from other sensors, such as the MAP sensor 30 and the RPM sensor 33. Using the mass of intake air per engine revolution, the ECM 11 then determines the injector pulse width warranted by the current operating conditions. The ECM 11 continually adjusts the injector pulse width to correct for changes in various parameters, such as in the readings taken from the TPS, IAT, O.sub.2 and WRAF sensors, so as to maintain as closely as possible the target air-fuel ratio. At a given angle in the operational cycle of the crankshaft 24, the ECM 11 then directs drive circuit 45 to inject fuel from the fuel injector 17 for the time dictated by the injector pulse width.
Most four cycle engines are designed so that the intake and exhaust valves operate (i.e., open and close) in a fixed angular relationship to the angular position of the crankshaft. Many engines use only a single camshaft to control the opening and closing of the intake and exhaust valves. The newer, more advanced engines often use a dual cam arrangement, i.e., one camshaft to control the open/close timing of the intake valves and another camshaft to govern the open/close timing of the exhaust valves. In either case, each valve is biased by a spring to the closed position. Affixed to the camshaft(s) are as many cams as there are valves, with the cams for the intake valves being oriented at one angle and the cams for the exhaust valves being oriented at another angle. Because a camshaft rotates at half the speed of the crankshaft, each intake cam causes its corresponding intake valve to be open (against the bias of the spring) and closed at fixed intervals during the operational cycle of the crankshaft. Similarly, each exhaust cam causes its corresponding exhaust valve to be open and closed at fixed intervals.
The term "standard cam timing" refers to the opening and closing of the intake and/or exhaust valves at such fixed intervals. In engines that employ standard cam timing, a compromise must be reached between how smooth will the engine run at idle, how much torque will it be able to deliver at medium to high speeds, the toxicity of its emissions, aid how much fuel will the engine consume. It involves a decision as to when and how long the intake and exhaust valves should be open at the same time (i.e., valve overlap). The amount and phasing of valve overlap is a trade-off between stable idling and the amount of power that will be available at medium to high speeds. It also is a trade-off between engine performance, emissions and fuel economy.
The automotive industry is now investigating the use of variable cam timing (VCT) schemes to improve the overall performance of an engine without the strict compromises required by standard cam timing techniques. VCT allows the timing of the camshafts, and thus the opening and closing of the valves, to be optimized over a wider range of operating conditions. It offers the possibility of improved performance at medium to full loads coupled with reduced emissions and improvements in fuel economy.
Referring to FIG. 1, the opening and closing of intake valve 18 is controlled by a cam 70 attached to an intake camshaft 71. A cam 80 attached to a camshaft 81 likewise controls the opening and closing of exhaust valve 21. As the pistons reciprocate within their respective cylinders, the torque they impart to the crankshaft 24 via the connecting rods 23 also drives a timing pulley 60. Each camshaft at its end also has a pulley, with camshaft 71 having timing pulley 61 and camshaft 81 having timing pulley 62. A timing belt 63 connects the timing pullers 60, 61 and 62. Consequently, as the crankshaft 24 rotates, it also drives the camshafts 71 and 81, with the cams 70 and 80 thereon opening and closing the intake and exhaust valves 18 and 21 at predetermined angles in the operational cycle of the crankshaft 24. A crankshaft sensor 56 generates a set number of pulses (e.g., 58 pulses) for each rotation of the crankshaft 2,4. Similarly, there are two camshaft sensors 57 and 58. Each camshaft sensor 57 and 58 generates a set number of pulses (e.g., 4 pulses) for each rotation of its respective camshaft 71 and 81.
The dual cam engine shown in FIG. 1 has two continuously variable cam timing mechanisms 72 and 82, one for the intake valves and the other for the exhaust valves. Controlled by the ECM 11, each VCT mechanism enables its respective camshaft to be phase-shifted relative to the crankshaft 24 as a function of the conditions under which the engine 1 is operating. Also referred to as cam phasers, VCT mechanisms take a variety of forms such as the vane type or helical gear type cam phasers. The latter is discussed below for illustrative purposes.
Situated between the camshaft 71 and the timing pulley 61, the intake VCT mechanism 72 turns the camshaft 71 and timing pulley 61 relative to each other. More specifically, the intake cam phaser 72 uses the camshaft 71 and timing pulley 61 as external gears and interconnects them via an intermediate helical gear. Through drive circuit 46, the ECM 11 controls a valve 73 that affects the hydraulic pressure acting upon the helical gear. It also uses feedback from sensors 56 and 57 to monitor the angular relationship between the crankshaft 24 and the intake camshaft 71. By changing the hydraulic pressure via valve 73, the ECM 11 can move the helical gear axially, and thus alter the angular relationship between the intake camshaft 71 and the timing pulley 61 as well as the crankshaft 24. In doing so, the ECM 11 can adjust the open/close timing of the intake valve 18.
The exhaust VCT mechanism 82 is situated between the camshaft 81 and the timing pulley 62. Like the intake cam phaser 72, the exhaust cam phaser 82 uses the camshaft 81 and timing pulley 62 as external gears and interconnects them via an intermediate helical gear. Through drive circuit 47, the ECM 11 controls a valve 83 that affects the hydraulic pressure acting upon this helical gear. It also uses feedback from sensors 56 and 58 to monitor the angular relationship between the crankshaft 24 and the exhaust camshaft 81. By changing the hydraulic pressure via valve 83, the ECM 11 can move this helical gear axially, and thus alter the angular relationship between the exhaust camshaft 81 and the timing pulley 62 as well as the crankshaft 24. In doing so, the ECM 11 can adjust the open/close timing of the exhaust valve 21.
Using VCT mechanisms, the open/close timing of the intake and exhaust valves 18 and 21 can be optimized to improve the overall performance of the engine 1. In dual overhead cam (DOHC) engines, there are four possible types of VCT: (1) phasing only the intake cam (Intake Only); (2) phasing only the exhaust cam (Exhaust Only); (3) phasing the intake and exhaust cams equally (Dual Equal); and (4) phasing the intake and exhaust cams independently (Dual Independent). The Dual Equal strategy is also applicable to single overhead cam (SOHC) engines.
It is well known that use of a VCT mechanism on only the intake camshaft 71 improves engine operation. This involves varying the open/close timing of the intake valve 18, as compared to standard cam timing, when the engine 1 is operating at part load. For example, by advancing the opening of the intake valve 18, the valve overlap is extended into the exhaust stroke. This means that the intake valve 18 starts to open near the end of the exhaust stroke. Viewing FIG. 1, this allows the piston 20, in its upward exhaust stroke, to push a small amount of the exhaust gases back into the intake duct 12. On the subsequent (downward) intake stroke, this exhaust gas is then re-ingested into the cylinder 13 for combustion with the fuel-air mixture. By advancing the closing of the intake valve 18, the intake valve 18 closes earlier in the compression stroke. This means that less of the fuel-air mixture is pushed back into the intake duct 12, thereby enabling more power to be produced during combustion.
The benefits of intake cam phasing are well known. First, it reduces NO.sub.x emissions. This is due to what is referred to as increased residual dilution. The re-ingested exhaust gases (i.e., the diluent) lowers the temperature at which combustion occurs, thereby reducing the amount of NO.sub.x emissions. The extent of the NO.sub.x reduction depends on the load and speed of the engine. Second, it reduces HC emissions. The last portion of the exhaust gases ejected from cylinder 13 during the exhaust stroke is rich in unburned HC. It is this portion of the exhaust gases that is re-ingested during the intake stroke and subsequently burned. Advanced intake cam timing also increases the torque output by the engine at medium to high speeds, improves fuel economy and enables the engine to be operated more stably at idle.
It is also well known that use of a VCT mechanism on only the exhaust camshaft 81 has a significant effect on emissions. This involves varying the close/open timing of the exhaust valve 21, as compared to standard cam timing, when the engine 1 is operating at part load. For example, by delaying the closing of the exhaust valve 21, the valve overlap is extended into the intake stroke. This means that the exhaust valve 21 stays open at the start of the intake stroke. Viewing FIG. 1, this allows the piston 20, in its downward intake stroke, to draw a small amount of the exhaust gases from the exhaust manifold 25 not only back into the cylinder 13 but also into the intake duct 12 due to vacuum. Along with the fuel-air mixture, this exhaust gas is then burned in the combustion chamber 19 during the combustion cycle.
The benefits of exhaust cam phasing are well known. First, it also reduces NO.sub.x emissions due to increased residual dilution. Second, HC emissions are reduced because the HC-rich portion of the exhaust gases is drawn back into the cylinder 13. Delayed exhaust cam timing also improves fuel economy and enables the engine to be operated more stably at idle. In addition, exhaust cam phasing can be used as a substitute for an external exhaust gas recirculation (EGR) system, as it performs the same function. The cost of equipping a vehicle with an exhaust cam phaser can be less than that for a conventional EGR system.
U.S. Pat. No. 5,713,317 to Yoshioka describes a method of controlling a VCT mechanism through which to vary the open/close timing of a valve. It purports to optimize the valve timing so as to improve the output of the engine at high altitudes while it is operating under high loads. It also purports to reduce the fuel consumption and emissions of the engine at high altitudes as it operates under low to medium loads. The method essentially controls the amount of residual dilution (i.e., re-ingested exhaust gases). In doing so, the Yoshioka reference teaches advance of the intake valve only, in a way that attempts to compensate for the effects of altitude.
U.S. Pat. No. 5,755,202 to Stefanopoulou et al. teaches the use of a Dual Equal VCT strategy on a vehicle equipped with ETC and a torque based engine control system. According to the method, the range of torque that can be demanded of the engine is divided into five regions, namely, negligible, small, moderate, high and maximum. The engine control system chooses the particular cam timing schedule to use according to the region into which the actual torque demand falls. For example, in the negligible torque region, standard cam timing is used to maintain the engine at, a stable idle. In the small torque region, the timing scheme falls between standard and fully retarded cam phasing, with the exact timing dependent on the magnitude of the torque demand. In the moderate torque region, fully retarded cam timing is used, and the throttle position is adjusted to meet the torque demand. It is in this region that the benefits of VCT (e.g., reduced feed gas (NO.sub.x and HC) emissions and improved fuel economy) are most fully realized. In the high torque region, the timing scheme falls between fully retarded and standard cam phasing, with the exact timing dependent on the magnitude of the torque demand. Here, the throttle position is held constant. It is in this region that the cam scheduling is relaxed to best meet the torque demand. In the maximum torque region, the schedule reverts to standard cam timing. This enables the engine control system to satisfy the torque demanded by the driver.
The disadvantage of the Stefanopoulou et al. system lies in the way it selects the cam timing mode according to the region into which the actual torque demand falls. Such transitions between modes can lead not only to discontinuities during throttle transient maneuvers but also to increased calibration time. It is therefore desirable to devise a strategy that employs variable cam timing in a way that assures smoother transitions in torque while achieving even lower feed gas emissions and increased torque.